Parasitology: Is the study of parasites, their hosts, and the relationship between them. As a biological discipline, the scope of parasitology is not determined by the organism or environment in question, but by their way of life.

It is a science which deals with organisms that take up their residence, temporarily or permanently, in or on other organisms for the purpose of procuring nourishment. The host may also provide shelter and protection

A Parasite: Is an organism that lives in or on another organism called ‘host’, usually larger than itself, from which it obtains nourishment and which may or may not harm the host.

Usually the host also provides shelter and protection to the parasite

Parasitism: Is any life association in which one species depends on another species. But usually refers to an association in which one organism, the ‘host’ is injured, to some degree, by the activities of the other, the ‘parasite’.

A host: Is an organism that harbours a parasite

Infestation: Is the invasion of parasites in the epithelial surfaces of the outer body surfaces and body cavities.

Infection: Is the invasion of parasites in the body tissues and organs.

Symbiosis

The word symbiosis is derived from the greek word ‘symbioum’ meaning ‘living together’

Symbiosis is the relationship in which the two associates cannot exist independently

They provide protection, nutrition or other advantages to one another

Example normal flora in the GIT produces Vitamin K and Vitamin B-complex, while the host provides shelter

The word is derived from the Latin word ‘mutuus’ meaning ‘exchanged’

Is the relationship in which both associates benefit from each other.

Commensalism is the relationship where the parasite derives benefit without reciprocation but also without injury to the host, e.g. Entamoeba coli in the intestine and Entamoeba gingivalis in the buccal cavity.

In parasitism, the host is injured by the parasite: i.e. pathogenesis and pathology occur.

The majority of parasites of man fall into this group.

Examples

Plasmodium causing malaria

Entamoeba histolytica causing amoebic dysentery

Trypanosoma gambiense causing trypanosomiasis

Wuchereria bancrofti causing hydrocoele and elephantiasis

Ascaris lumbricoides causing intestinal worm infection

Ectoparasite

This is a parasite that lives on the surface of the host and its presence is referred to as infestation.

Examples

Ascaris lumbricoides causing worm infestation in the gastro intestinal tract

Sacorptes scabei causing the disease called scabies which is a form of dermatitis

This lives inside the host, i.e. invades various organs and tissues giving rise to an infection Examples

Plasmodium falciparum causing malaria invades the liver and red blood cells

Hookworm Necator americanus causes hookworm anaemia

These are parasites that cannot exist when separated from the host

Example filariae, malaria, and trypanosomes.

Facultative Parasites

These can exist away from the host, they can exist both as parasites and as free-living organisms Example Strongyloides stercoralis

These are not normal parasites of man but enter or attach to the body and cause parasitism in man Example

Dirofilaria immitis; the dog heart filaria

Toxocara canis; an ascarid of dogs that causes visceral larva migrans in man

These are usually commensals or non-pathogenic organisms

They invade the body and cause disease when the host’s immune status is compromised

Examples Toxoplasma gondii, Cryptosporidium parvum, Isospora belli, Microspora

Larval Parasites

Larval Parasites are pathogenic parasites which infect man and cause pathology during their larval stage of development.

Their adults may be parasites of other animals, man or they may be free-living.

Example Echinoccocus granulosus, Taenia solium, Spirometra mansonoides, Multiceps multiceps.

Types of Hosts

Definitive/Final Host

This is the host that harbours the adult stage or the sexual stages of the parasite.

It is the host in which the mature or most developed form of the parasite occurs.

Example man is the definitive host of trypanosomes that cause African sleeping sickness.

Intermediate Host

Harbours the larval or the asexual form of the parasite

Example cow is intermediate host for T. saginata, Man is an intermediate host of malaria parasites.

Host in which the parasite does not undergo any development but in which it ramains alive and infective to another host.

Example plerocercoides of Diphyllobothrium latum can be carried from smaller fish to larger fish which are then consumed by humans and they proceed to develop into adults in humans

Harbour the same parasites as man and serves as source of infection to man

Example rats with Trichinella spiralis, dogs harbour infection with Leishmania,

Armadillos harbour the parasite Trypanosoma cruzi that causes Chaggas disease.

Host Specificity

Most parasites develop only in a restricted range of host species, i.e. parasite exhibit a varying degree of host specificity; some infecting only a single host, e.g. Taenia solium adult can only develop in humans. But Trichinella spiralis is able to mature in almost any mammal.

Mechanical Vectors

Are vectors which assist in the transfer of parasite forms between host but are not essential in the life cycle of the parasite, i.e. no parasite development occurs in such vectors.

Examples of mechanical vectors are Musca domestica which can transfer cysts of Entamoeba histolytica from faeces to food that is eaten by man.

These are vectors in which there is biological development of the parasite before it can be transmitted

Example mosquitoes are biological vectors of malaria parasites, filarial worms, and yellow fever virus

Parasites of medical importance can be classified according to two criteria

Biological nature (taxonomic)

Location in human body

Parasites are classified into kingdom, phylum, class, order, family, genus and species.

The parasites of humans in the kingdom protozoa are classified under four phyla: Sarcomastigophora, Apicomplexa, Ciliophora and Microspora.

The parasitic worms, or helminthes, of human belong to two phyla; platyhelminthes (flatworms) and nemathelminthes (roundworms).

Within these great assemblages (phyla) are found the important human parasites, conveniently listed as subphyla

Sarcomastigophora

The important subphyla include

Sarcodina (amoebas)

These are ameboid in shape and the species infecting humans include Entamoeba

histolyitica and Naegleria floweri.

These are flagellates which have one or more flagella.

In some cases they contain an undulating membrane (example in trypanosomes).

Mastigophora include intestinal and genitourinary flagellates (Giardia, Trichomonas, Chilomastix) and blood and tissue flagellates (Trypanosoma, Leishmania).

The important subphyla include Sporozoa,

SPOROZOA

Undergo a complex life cycle with alternating sexual and asexual reproductive phases usually involving two different hosts (arthropods and vertebrate) and the number of parasites multiplies in each host

Sporozoa contains two classes of medical importance which include Coccidia, Hematozoa

The class Coccidia contains the human parasites like Isospora and Toxoplasma species

The class Haematozoa contains the blood sporozoans like the Plasmodium species.

Ciliophora (containing the ciliates)

Ciliophora are complex protozoa bearing cilia distributed in rows or patches.

They bear two kinds of nuclei in each individual.

The medical important cilliophora and only human parasite representative of this group include

Balantidium coli (a giant intestinal ciliate of humans and pigs).

Microspora

This is a distinctive group, formerly listed with the protozoa, often within the sporozoa.

It is now considered a separate phylum

It comprises microsporidians which are seen as opportunistic parasites of immuno suppressed hosts (microsporidiasis)

The Parasitic Worms, or Helminthes, of Medical Importance

These belongs to two phyla: Platyhelminthes and Nemathelminthes

Platyhelminthes (flatworms)

Lack a true body cavity (coelom) and are characteristically flat in dorsoventral section

There are two medically important classes which include

Cestoda (tapeworms)

These are band-like and segmented Examples of the genera of medical importance include Diphyllobothrium, Spirometra, Taenia, Echinococcus, Hymenolepis, and Dipylidium

Flukes are typically leaf-shaped

Flukes and tapeworms of humans are hermaphroditic except schistosomes Schistosomes are cylindrical and worm like

Examples of trematodes of medical importance include Schistosoma,Paragonimus, Clonorchis, Opisthorchis, Heterophyes, Metagonimus, Fasciolopsis,and Fasciola

These have separate-sexes and unsegmented roundworms

They have body cavities

They have complete digestive, reproductive and nervous systems

They include many parasitic species that infect humans.

Examples Necator americanus/ Ankylostoma duodenale (hook worm), Enterobius vermicularis, Trichuris trichiura, Ascaris lumbricoides and Strongiloides stercoralis,

Wuchereria bancrofti, Onchocerca volvulus and Dipetalonema persitans.

An essential procedure in diagnosis of many helminthic infections is microscopic recognition of ova or larvae in faeces, urine, blood, or tissues

Parasites that cause infection to human are classified according to location they infect.

This gives the four major categories of parasites

Intestinal (gut) parasites

Tissue parasites

Blood parasites

Genital urinary parasite

Parasitology is a science which deals with organisms that take up their residence, temporarily or permanently, in or on, other organisms for the purpose of procuring nourishment and shelter

Parasitism is the relationship between the parasite(small) and the host(large)

Parasites may be endo or ectoparasite

Classification of parasites of medical importance is done by biological nature (taxonomic classification) and site of residence within the body

Major groups of parasites that infect man include single celled parasites called protozoa and multicellular parasitic worms which are called metazoa or helminthes

What is parasitism?

List types of hosts and vectors.

How are the parasites of medical importance classified?

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